If it’s Tuesday, that means it’s primary day in an election season that is increasingly feeling like “Groundhog Day.”

And Catholics, according to this report, are playing a huge role in one of the campaigns:

White Roman Catholics have become a key base for Sen. Hillary Clinton in her quest for the Democratic presidential nomination and one she needs to win by huge margin, a conference on faith and politics heard on Monday.

William Galston of the Brookings Institution presented one of the most detailed portraits to date of the Catholic vote in last month’s Pennsylvania primary, which the New York senator won to keep her battle with Illinois Sen. Barack Obama alive.

It is a portrait that has relevance as she campaigns before crucial primaries on Tuesday in North Carolina and Indiana.

“Catholics have emerged in the course of this primary season as part of Clinton’s base that she not only has to win but win with a strong majority,” Galston said.

About a quarter of Americans consider themselves Catholic.

Galston said that, while Indiana was less Catholic than many other states, 18 percent of people there still claimed membership in that faith and the percentage of Catholic Democratic primary voters there would probably be even higher.

“So doing well (with Catholics) will give her an edge over Sen. Obama in Indiana,” he told Reuters after presenting his analysis at “The Faith Angle” conference in Key West, Florida, organized by the Pew Forum on Religion & Public Life.

The share of Catholic voters drops to about 9 percent in North Carolina, making it unlikely the constituency will give Clinton a significant boost there.

Familiarity with Catholic language and sensibilities certainly works in Clinton’s favor. When she said a few years ago that every abortion is a tragedy, some of her strongest supporters were outraged—and it was more of a Sister Souljah moment than Obama has ever had. In fact, what he’s said on the subject has had the opposite effect. At a campaign rally in Pennsylvania, Obama said that when providing information about sex to his own daughters, “I am going to teach them first of all about values and morals. But if they make a mistake, I don’t want them punished with a baby.” For even mildly pro-life voters, using the word “punished” in that context was at least as unfortunate as describing believers as “bitter.” “He’s got more missteps on the life issues than she does,” said a lobbyist in Washington, D.C., who took leaves from work to volunteer for Bill Clinton’s and Al Gore’s presidential campaigns but who wrote in Bob Casey for president in ’04 and is drifting away from the Democrats over the abortion issue. “That comment about not wanting to punish his daughter with a baby was all over the life blogs, so even though their positions are the same,” language does matter, as does communicating respect for Democrats with a dissenting view on abortion rights. “If you’re a Democrat desperate to come back to the party, you might be more comfortable with her.”

Though “Saturday Night Live” wouldn’t seem to be in the vanguard of Catholic thought, Tina Fey may have been onto something with her “Bitch Is the New Black” comparison of Hillary to a cranky but proficient old nun: “Bitches get stuff done; that’s why Catholic schools use nuns as teachers and not priests. They’re mean … and they sleep on cots, and they’re allowed to hit you. And at the end of the school year, you hated those bitches. But you knew the capitol of Vermont.”

The ’04 Casey voter says nun-run Catholic schools turned out a lot of good feminists: “Older Catholics with exposure to nuns in school may be more comfortable with women in positions of authority.” And my one Catholic friend who does back Hillary thought the opposite experience also worked in Clinton’s favor: “Don’t you think Catholic women are tired of not seeing women in authority positions?” In Pennsylvania, where she has family, “My aunt and all her friends went for Hillary. There was a lot of, ‘You go, girl.’ ” And for her, part of the appeal is the general impression that Clinton seems more devoted to her faith than either Obama or John McCain. “She just seems the most religious of the three—particularly now that Obama’s thrown his preacher under the bus.”

There’s more at the Slate link, including the surprising news (to me, anyway) that some of Hillary’s religious philosophy was influenced by Henri Nouwen.